This invention relates to a method and an apparatus for treating the fluorescent substances used in illumination lamps, cathode ray tubes for television receivers and the like.
Such fluorescent substances should have excellent luminous efficacy and a long life. However, the luminous efficacy of conventional fluorescent substances decreases with time as a result of short wavelength ultraviolet irradiation. Accordingly, the intensity of fluorescent lamps gradually decreases during use. Particularly, the fluorescent substances containing halogen significantly exhibit such a tendency.
U.S. Pat. No. 2,965,786 describes a fluorescent substance which contains between about 0.01 and 0.25 gram-atoms of cadmium per 3.00 gram-moles of phosphate. However, ultraviolet irradiation still lowers the luminous efficacy by too great a degree.
Japanese patent Disclosure No. 151555/77 and No. 151558/77 describe a method for treating with plasma the surface of a fluorescent layer formed on a substrate. However, in this method, the fluorescent layer is placed in a plasma-producing region where intense short wavelength ultraviolet irradiation is produced. The fluorescent layer is thus damaged by the ultraviolet irradiation and ion bombardment.
Also, the known electrostatic coating technique is typically employed to apply the fluorescent coating to fluorescent lamps and high voltage fluorescent mercury lamps. With the electrostatic coating technique, cohesion between powder particles of the fluorescent substance should be very small and fluidity and dispersion of the fluorescent powder should be large. A method which adds a small amount of almina or silica to the fluorescent substance so as to reduce the cohesion and increase the fluidity of the fluorescent substance is known. However, almina and silica lower the luminous efficacy of the fluorescent substance, and the life of the fluorescent lamp is shortened.